Twelve-year laboratory tests of rebar reinforced concrete beams partially submerged in artificial seawater have confirmed that steel corrosion may occur a few months after immersion and may continue for many years. An average steel potential of-71 mV (SCE) was established during the first few months of immersion. Pitting corrosion, marked by a sharp shift in potential of-200 to -300 mV, occurred when chlorides in sufficient concentration reached the rebar surface. Oxygen in air bubbles on the surface provided the cathodic reactant. The average potential reached a maximum negative potential of-547 mV (SCE) after four years immersion and subsequently increased to -360 mV after 12 years. The potential records are marked throughout by frequent spikes varying in amplitude from a fraction of a millivolt to more than 100 mV. It is believed the spikes reflect continuing competition between passivation in the highly alkaline pH of hydrated cement and corrosion in the acidic pH of active anodes. The tests have demonstrated that passive steel potentials are at least 200 mV more positive than corroding potentials, although this separation may not be evident in field measurements because of potential averaging effects.
INTRODUCTION Concrete piles in the sea rarely crack or show signs of steel corrosion below the level where the concrete is constantly saturated with seawater. The usual explanation is that oxygen required for corrosion is excluded in the saturated zone. The laboratory tests reported herein were made to obtain a more complete explanation of what occurs in this zone, to identify trends in steel potential, and to relate changes in potential to electrochemical processes involved.
In a previous research program, tests similar to those described were made with both smooth steel bars and deformed bars (rebars) and various saline solutions including several different concentrations of artificial seawater.1 Some results of this earlier program are presented and discussed in this report.
Passivation and corrosion
Iron atoms on steel surfaces in chloride-contaminated concrete are continuously subjected to the competing electrochemical reactions of passivation and corrosion. A passivation event occurs when three hydroxyl ions (OH-) react with an iron atom to form a molecule of gamma ferric hydroxide. The probable half cell reaction is
Fe + 3OH---> yFeOOH + H20 + 3e- (1)
A corrosion event occurs when two chloride ions (C1-) react with an iron atom to form highly soluble ferrous chloride, the net half cell reaction being:
F e - - > Fe z+ + 2e- (2)
Neither half cell will exist unless the electrons released are utilized simultaneously in cathodic reactions. concrete the cathodic half cell reaction is hydrolysis of oxygen:
1/2 02 + H20+ 2 e - - - > 2OH- (3)
Dissolved oxygen will support the reaction of Equation 1, but Equation 2 will not take place unless oxygen is present on steel surfaces in the form of visible air bubbles. This can be demonstrated by immersing a polished steel bar and some concrete fragments in a jar of seawater (Figure 1). Alkalinity released by the concrete will prevent corrosion except where small air bubbles adhere to the surface as the bar is immersed. Pinhead-size anodes quickly develop at these locations, but corrosion stops when oxygen in the bubbles is exhausted.
The probability of corrosion
Equations 1 and 2 suggest that passivation and corrosion are equally probable when the electrolyte contains three hydroxyl ions for every two chloride ions, and for steel corrosion to occur, the C1-/OH-ratio must exceed a 'threshold' value of 2/3. The validity of this corrosion threshold concept was first demonstrated
